image-artifact-names.bbclass: add IMAGE_VERSION_SUFFIX_DATETIME which uses SOURCE_DAT...

image-artifact-names.bbclass: add IMAGE_VERSION_SUFFIX_DATETIME which uses SOURCE_DATE_EPOCH

* since https://git.openembedded.org/openembedded-core/diff/meta/classes/image-artifact-names.bbclass?id=abb0671d2cebfd7e8df94796404bbe9c7f961058
  which removed the
  bb.data.inherits_class('reproducible_build', d)
  condition this was already applied in all the builds which used DATETIME, so we
  can move it to the default value directly and DISTRO configs than can choose
  to use IMAGE_VERSION_SUFFIX_DATETIME as they want

[YOCTO #12937]

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>